Some gems have second thoughts about their color as they move from one lighting environment to another - usually switching from red or purple in incandescent to green or blue in fluorescent light. There aren't many of these color-change gems. In fact, being of two minds about hue is a character trait of just four species: chrysoberyl, sapphire, spinel, and garnet. (Other gems like andalusite and sphene which exhibit hue-changes in the same lighting environment are known as color-shift gems and deserve their own separate survey story.)
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